Plaintext is easily readable or, to be more accurate,
decrypted/unencrypted text.
This is opposed to ciphertext, which is encrypted.
If you used an encryption algorithm to encode your message of
"Hello, there" to "4hgu28fhdjf83291".
The plaintext form would be: "Hello, there"
The ciphertext form would be: "4hgu28fhdjf83291"